1. Field of the Invention
The present invention relates to an image adjusting method for an output image in an image forming apparatus, and the image forming apparatus.
2. Description of the Related Art
In general, image forming apparatuses such as a color copying machine and color printer need to always output stable images.
In practice, however, the density of each color varies and color misregistration occurs owing to changes in ambient temperature and moisture, deterioration of the image forming system over time, and the like.
To prevent this, there is known an image forming apparatus which adjusts the process conditions of each image forming station by forming a pattern image for detecting the image density and by optically detecting the density of the pattern image. This adjustment is called density adjustment processing. There is also known an image forming apparatus which adjusts a reflecting mirror inserted in the optical laser path and adjusts the image write timing by forming a pattern image for adjusting the image forming position and by optically detecting the position of the pattern image. This adjustment is called position adjustment processing. Conventionally, whether to perform these adjusting processes is determined depending on count data such as the number of prints or the total number (video count) of data “1” among image data. This technique can always output stable images (e.g., see U.S. Pat. No. 6,937,826).
However, an image forming sequence, which is fundamental in image forming processing, must be interrupted when executing the above-described processing of forming an adjustment pattern, optically detecting the pattern image, and feeding back the detection result in order to adjust various image forming conditions. Because image forming processing is interrupted during the adjustment, the user productivity decreases; that is, the image forming requires a long time.
Recently, as the speeds of a color copying machine, color printer, and the like increases, a decrease in productivity by a down sequence becomes an issue in addition to the decrease in productivity by image adjustment.
The down sequence is to print by widening the sheet feeding interval in order to prevent the temperature of a fixing unit from becoming lower than a temperature at which the fixing characteristic can be maintained. Such temperature drop occurs when, for example, a sheet deprives heat of the fixing unit during continuous printing. As the printer becomes faster, the amount of heat deprived by sheets of the fixing unit becomes larger than that of heat applied from the heater to the fixing unit. The temperature of the portion of the fixing unit where paper passes readily drops, and the printer is prone to shift to the down sequence.
Another example of the down sequence is to print by widening the sheet feeding interval in order to prevent a temperature in an area of the fixing unit where no sheet passes, from exceeding a specific temperature necessary to ensure the quality of the surface of the fixing roller. Such temperature rise occurs during continuous printing on sheets at a size smaller than the width of the fixing unit. When the amount of heat applied from the heater increases along with speed-up of printers, the temperature difference between the portions where paper passes and paper does not pass becomes large during printing on small-size sheets. The temperature at the portion where paper does not pass readily rises, so the printer is prone to shift to the down sequence.
As described above, the user suffers two productivity decrease factors, i.e., the conventional image adjustment and the down sequence.